Steels having improved machinability and method for manufacturing



United States Patent 3,250,647 STEELS HAVING IMPROVED MACHINABILITY AND METHOD FOR MANUFACTURING Elliot S. Nachtman, Evanston, Ill., assignor to La Salle Steel Company, Hammond, Ind., a corporation of Delaware No Drawing. Original application Aug. '5, 1960, Ser. No. 47,606, now Patent-N0. 3,094,440, dated June 18, 1963. Divided and this application June 17, 1963, Ser. No.

8 Claims. (Cl. 148-4) This invention relates to plain carbon and alloy steels and to a method of improving the machinability thereof. It is concerned more particularly with a method of improving the machinability of plain carbon and alloy steels accompanied with improvements also in physical and mechanical properties of the steel.

This is a division of my copending application Ser. No. 47,606, filed August 5, 1960, entitled Steels Having Improved Machinability and Method for Manufacturing, now Patent No. 3,094,440.

It is an object of this invention to produce and to provide a method for producing plain carbon and alloy steels in which the machinability is improved by means easily and simply carried outduring normal steel making and it is a related object to improve the machinability of steel with improvement also in other physical and meof steels may be afforded by incorporating into the steel priorto solidification an additive in the form of certain 'inorganic compounds of the above metals, particularly their sulfides, sulfates, sulfites, tellurides vor selenides. These compounds arefurther enhanced in their ability to improve the machinability of non-austenitic steels by combining therewith, in the finished steel, sulphur, phosphorus and nitrogen. These compounds are for purposes of simplicity referred to hereinafter as additives? One of the suprising features of the invention resides in the relatively small quantities of the described inorganic compounds of copper, cadmium, zinc, cobalt, mercury and tin required to improve the machinability of certain types of steels. In the case of zinc sulfide, as little as 0.01 percent (expressed as Zn) gives good results although it is preferred to use from 0.02 percent by weight of this compound as well as the other compounds up to their solubility .inthe. particular steel treated.

Improvements .in machinabilitylof steel by -the use of the compounds .of the invention are noticeable with low carbonsteel (less than 0.25 percent-carbon as well as with medium carbon steels (0.25-0.50 percent carbon) and high carbon steels (above 0.5 percent carbon). A typical treated steel .of the invention has the following composition as shown below in Table I. Also shown is the same steel without treatment. The additive used was zinc sulfide.

chanical properties of the steel, such as strength, resist- TABLE I ance to wear, resistance to corrosion and toughness.

Numerous attempts have been made in the past to im-' Steel militant), n (Treated),

. percent percent prove the machinabihty of such plain carbon and alloy steels. To the present, lead, selenium, sulphur and bis- Carbon 046 047 muth have been found to provide noticeable improve- Manganese, n 1158 1:54 ments in the machinability of steel when added to provide g g gf g g-ggg a certain concentration in the steel, but certain undesir- Silicon I i123 6.23 able conditions have been associated with their use. For gfi g e 8122 8-22 example, improved machinability has been secured bythe Mplybdenuin 0: 03 0: 03 addition of lead in the manufacture of leaded steels, but ;;j Not g gg 8:8; since lead is insoluble in the steel, it isdifficult to achieve 40 0. 005 the desirable uniform distribution of the lead in the steel.

In addition, lead releases toxic fumes under the processing conditions with the result that extensive precautions must be taken in the use thereof.

Similarly, the distribution of sulphur in the desiredconcentration in the steel is also diflicult to control and unless combined with manganese in substantial amounts, the sulphur will produce an undesirable condition in the steel during rolling, generally referred to as hot short ness. Bismuth and selenium have been used on a minor scale in stainless steel for the purpose of improving machinability. However, the additional cost occasioned by the use of such materials in desirable amounts is a deterrent to their adoption. In addition, these various elements which have been added to steels to improve machinability in many cases cause an undesirable loss in some of the mechanical and physical properties of the steel.

It has been found in accordance with the invention described and claimed in my copending application Ser. No. 652,359, filed April 12, 1957, for Method forlmproving the Machinability of Steel, which is a continuation of Ser. No. 459,527, filed September 30, 1954, now US. Patent No. 2,789,069, that the machinability of plain carbon and alloy steels can be improved materially by the to tool cutting tests.

To demonstrate the improvements in machinability achieved by utilizingthe compounds of the invent-ion, the following is presented by way of example.

Example Samples of the steels shown in Table I were subjected A measure of the relative machinability was obtained by measuring the wear on the tools after of the parts being machined had been cut under substantially identical conditions.

The force values for a 0.050'inch depth of cut a feed of .00028 inch per revolution, a cutting speed of 200 feet per minute and no cutting fluid was used. The surface finish results were obtained from bars 5 inches long and tapered .005 inch on the radius along the 5 inch length. The width of cut was 0.01 inch in the feed direction while the undeformed chip thickness Was variable from 0 to 0.01 inch. This procedure makes it possible to measure the finish without the inclusion of feed marks. The cutting speed was 200 f.p.m. for the finish tests and a dry tool was used. The elfective rake angle was 10 in both the force and finish tests.

The finish for steel II is far better than that for steel I. The cutting forces for steel II are unusually low whereas the cutting force values for steel I are relatively high.

Zinc selenide Zinc sulfate Zinc sulfide Zinc sulfite Zinc telluride Cobaltic sulfate Cobaltous sulfate Cobaltic sulfide Mercurous sulfide Mercuric selenide Stannic sulfate Stannous sulfate Stannic sulfide Stannous sulfide Stannic selenide Stannous telluride Cobaltous sulfide Cadmium selenide' Cobaltous sulfite Cadmium sulfide Cobaltous selenide Cadmium sulfate Mercuric sulfate Cadmium sulfite Mercurous sulfate Cadmium telluride Mercuric sulfide In the past it has been recognized that residual metals, such as nickel and chromium, have caused marked depreciation in the machinability of steels even when present in small amounts. Contrary to the accepted limitation,

it has beenfound that the deleterious effects of the residual metal on the machinability of steels is reduced by the use of the compounds of this invention. While it is preferred to make use of a steel in which such residual metals are not present, since the metals of the invention are capable then of maximum use, it has been found that the presence of residual metals does not destroy the machinability characteristics of the steel in the presence of the additives of this invention even though less improvement is secured by the use of such additives, nevertheless, in a steel containing the additives in combination with the residual metals provides for better machinability than the same steel withavailable from the use of the additives of the invention in plain carbon steels and alloy steels can be extended by the addition of other elements with the additives, such for example as phosphorus, sulphur and nitrogen. To some extent, these other elements have been used before because of their'beneficial effects on machinability but the impr'overnent which is secured in a system which makes use both of the additive and sulphur together in plain carbon and alloy steels exceeds that which-might be expected by way of aggregation to the extent that improvement in machinability is indicative more of a synergistic effect between these materials in steel. These unexpected results are secured when sulphur is present in amounts ranging from 0.01 to 1.0 percent by weight in the steel and when the additives are present in an amount within the range of 0.06 up to their solubility in steel when in a solid state but in an amount not to exceed 0.6 percent by weight.

When phosphorus is present, the best results are secured when the concentration of phosphorus is within the range 0.01 to 0.2 percent by weight, and the additives are present within an amount ranging from 0.06 to 0.6 percent by weight or up to their saturation solubility when it is less than .6% by weight, and sulphur, when present, is present in an amount less than 1.0 percent by weight. Nitrogen in amounts greater than 0.001 percent by weight also improves the machinability of steel when present with the additives alone or in a system which includes the additives and one or more other elements such as sulphur, phosphorus and nitrogen.

Since many of the additives used in the practice of this invention contain quantities of sulphur, it will be understood that the amount of sulphur expressed above is intended to include the amount of sulphur added to the steel by way of the additive of the invention; that is to say, the sulphur content of the additives is included in the total amount of sulphur additives when such are used in combination with the additives in the practices of the invention.

In the manufacture of steels and other iron base alloys, the additives may be introduced into the metal when in the furnace, ladle or mold. It is an important concept of this invention that when the additives are used to treat steel, normal operating procedures may be employed.

Another important concept of this invention resides in the further discovery that the machinability of steel, for example as measured by the cutting tests previously described, especially steels having the additives in the amounts described, show unexpectedly large improvements in machinability when cold worked. As used herein, the term cold working is meant to include deformation of the steel, as by cold drawing or rolling at room temperature or at elevated temperatures, such as described in US. Patent No. 2,767,835; US. Patent No. 2,767,836;

US. Patent No. 2,767,837; and U.S. Patent No. 2,767,838. The most striking effects by way of improved machinability by cold working become apparent with steels containing 0.06 percent carbon or less. Particularly outstanding results are secured with steels having 010 to 0.25 percent carbon. To the present, such improvements in machinability are maximized by cold working the steel by reduction in excess of 15 percent in cross-sectional area and generally with reductions of between 20-50 percent depending upon the chemistry of the metal, the size of the raw material being cold worked, and in the case of drawing at elevated temperatures, the temperature of drawing.

When steel is drawn at about room temperature to achieve thedesired reduction, strain relieving by heat treatment at a temperature above 550 F. but below the lower critical temperature and preferably within the range of 550950 F. may be desirable. With steels having from 0.2 to 0.4 percent carbon, maximum improvement in ma'chinability is secured by taking heavy drafts which may be followed by strain relieving, especially when copper is present in such steels alone or in a system with sulphur, and with less than 0.15 residual metals Composed of nickel, chromium and molybdenum. When working is achieved by drawing at elevated temperatures as described in the aforementioned copending applications and issued patents, subsequent strain relieving steps by heat treatment become unnecessary.

While cold working has been found to be desirable to improve the machining characteristics of steel, the combination of cold working steels of thetype heretofore described containing the additive alone or containing the additives in a system with sulphur, and/or nitrogen, offers still further possibilities for providing high levels of machinability, together with improved strength, resistance to corrosion, toughness, wear and other mechanical and physical properties without introducing limitations in the processing characteristics of the steel.

The unexpected improvements in machinability secured by metal working is not fully understood. It has been found that steels, particularly those containing carbon in the range of from 0.10 to 0.25 percent, can appreciably be benefitted by heavy cold working. These beneficial effects with respect to machinability are enhanced when the steel contains the additives alone or in the presence of phosphorus, sulphur and/ or nitrogen. Whatever the reason, the machinability of steels and iron base alloys, as determined by the energy required for metal separation and by reduction in wear of the turning and forming tools has been found greatly to be improved by modification in the chemistry of the steel to include the additives as an essential component thereof, and further by working of the steel as by cold drawing to secure heavier than normal drafts. Such improvements in machinability have been instrumental in accelerating output of steel products by enabling processing with heavy feeds and higher speeds to increase the production rate while at the same time reducing the time required for replacement and repair of tools and parts. In addition to increased output at reduced costs, the presence of the additives to improve machinability has also provided improvements in other physical and mechanical properties as previously pointed out.

It will be understood that changes in details of formulation, methods or incorporation and processing of the various steels prepared in a manner to provide the characteristics of this invention may be made without departing from the spirit of the invention, especially as defined in the following claims.

I claim:

1. The metallurgical process for improving the machinability of steel comprising the steps of advancing the steel through a die to effect reduction in cross-sectional area wherein the steel advanced through the die is a free machining steel of the non-austenitic type to which a compound of cobalt has been incorporated as an additive in an amount within the range of 0.06 to 0.6% by weight, and in which the compound of cob-alt is selected from the group consisting of the sulfides, sulfates, sulfites, tellurides and selenides of cobalt.

2. The metallurgical process for improving the machinability of steel comprising the steps of advancing the steel through a die to effect reduction in cross-sectional area wherein the steel advanced through the die is a steel of the non-austenitic type to which a compound of cobalt has been incorporated as an additive in .an amount Within the range of 0.06 to 0.6% by weight, and in which the compound of cobalt is selected from the group consisting of the sulfides, sulfates, sulfites, tellurides and selenides of cobalt, and in which the steel contains less than 0.25% by weight residual metals selected from the group consisting of nickel, chromium, vanadium and molybdenum, and subsequently machining the steel to produce parts.

3. The metallurgical process for improving the machinability of steel comprising the steps of advancing the steel through a die to effect reduction in cross-sectional area wherein the steel advanced through the die is a free machining steel of the non-austenitic type to which a compound of cadmium has been incorporated as an additive in an amount within the range of 0.06 to 0.6% by weight, and in which the compound of cadmium is selected from the group consisting of the sulfides, sulfates, sulfites, tellurides and selenides of cadmium.

4. The metallurgical process for improving the machinability of steel comprising the steps of advancing the steel through a die to effect reduction in cross-sectional area wherein the steel advanced through the die is a steel of the non-austenitic type to which a compound of cadmium has been incorporated as an additive in an amount within the range of 0.06 to 0.6 percent by weight, and in which the compound of cadimum is selected from the group consisting of the sulfides, sulfates, sulfites, tellurides and selenides of cadmium, and in which the steel contains less than 0.25 percent by weight residual metals selected from the group consisting of nickel, chromium, vanadium and molybdenum, and subsequently machining the steel to produce parts.

5. The metallurgical process for improving the machinability of steel comprising the steps of advancing the steel through a die to effect reduction in cross-sectional area wherein the steel advanced through the die is a free machining steel of the non-austenitic type to which a compound of mercury has been incorporated as an additive in an amount within the range of 0.06 to 0.6% by weight, and in which the compound of mercury is selected from the group consisting of the sulfides, sulfates, sulfites, tellurides and selenides of mercury.

6. The metallurgical process for improving the machinability of steel comprising the steps of advancing the steel through a die to effect reduction in cross-sectional area wherein the steel advanced through the die is a steel of the non-austenitic type to which a compound of mercury has been incorporated as an additive in an amount within the range of 0.06 to 0.6 percent by weight, and in which the compound of mercury is selected from the group consisting of the sulfides, sulfates, sulfites, tellurides and selenides of mercury, and in which the steel contains less than 0.25 percent by weight residual metals selected from the group consisting of nickel, chromium, vanadium and molybdenum, and subsequently machining the steel to produce parts.

7. The metallurgical process for improving the machinability of steel comprising the steps of advancing the steel through a die to effect reduction in cross-sectional area wherein the steel advanced through the die is a free machining steel of the non-austenitic type to which a compound of tin has been incorporated as an additive in an amount within the range of 0.06 to 0.6% by weight, and in which the compound of tin is selected from the group consisting of the sulfides, sulfates, sulfites, tellurides and selenides of tin.

8. The metallurgical process for improving the machinability of steel comprising the steps of advancing the steel through a die to effect reduction in cross-sectional area wherein the steel advanced through the die is a steel of the non-austenitic type to which a compound of tin has been incorporated as an additive in an amount Within the range of 0.06 to 0.6 percent by weight, and in which the compound of tin is selected from the group consisting of the sulfides, sulfates, sulfites, tellurides and selenides of tin, and in which the steel contains less than 0.25 percent by weight residual metals selected from the group consisting of nickel, chromium, vanadium and molybdenum, and subsequently machining the steel to produce parts References Cited by the Examiner UNITED STATES PATENTS 2,789,069 4/ 1957 Nachtman.

DAVID L. RECK, Primary Examiner.

H. F. SAITO, Assistant Examiner. 

1. THE METALLURGICAL PROCESS FOR IMPROVING THE MACHINABILITY OF STEEL COMPRISING THE STEPS OF ADVANCING THE STEEL THROUGH A DIE TO EFFECT REDUCTION IN CROSS-SECTIONAL AREA WHEREIN THE STEEL ADVANCED THROUGH THE DIE IS A FREE MACHINING STEEL OF THE NON-AUSTENITIC TYPE TO WHICH A COMPOUND OF COBALT HAS BEEN INCORPORATED AS AN ADDITIVE IN AN AMOUNT WITHIN THE RANGE OF 0.06 TO 0.6% BY WEIGHT, AND IN WHICH THE COMPOUND OF COBALT IS SELECTED FROM THE GROUP CONSISTING OF THE SULFIDES, SULFATES, SULFITES, TELLURIDES AND SELENIDES OF COBALT. 